Those skilled in the arts of processing liquids desire to know how much air and/or other gases are entrapped and dissolved therein for a variety of reasons. Entrapped air can cause undesired foaming during processing, e.g. in papermaking and in the preparation of foodstuffs, and can result in disruption of film products, e.g. from paints. Entrained gases distort such processing parameters as density, making precise control of processes impossible. Those skilled in the art know that, generally, the more viscous a fluid being processed, the more difficult it is for any entrained air to escape from it and consequently the greater the amount of air bubbles likely to be accumulated therein. Also, as pressure on a fluid is lowered, dissolved air or other gas therein tends to leave solution and form bubbles in the fluid.
There are a number of instruments that are currently commercially available for measuring the air or gas content in a liquid. Such instruments include Valmet's COLORMAT, Mütek's GAS-60, Papec's PULSE))))AIR, Capella Technology's CAPTAIR, Anton-Paar's CARBO 2100 CO2 analyzer, and CyberMetrics' AIR TESTER.
Mütek's GAS-60, for instance, is said to be useful in the context of minimizing pinholes (voids) in papermaking processes. Pinholes develop when pressure is reduced and dissolved gases—which accumulate in the papermaking process due to mechanical effects and chemical and biological reactions—are released. The GAS 60 is installed on line and is used to determine the gas content of entrained and dissolved gases in pulp suspensions. Having determined gas content, process engineers are able to calculate how much (expensive) deaerating additive should be used, and thus to avoid unnecessarily increased manufacturing costs due to employing too much deaerating additive.
Papec's PULSE))))AIR_V3 is a sensor for the measurement of entrained air and gases in process fluids. It is said to be useful in the pulp and paper industry in connection with machine headboxes and white water systems, coatings, and brownstock washers, in the secondary fiber industry (for effluent treatment), in the paint industry, in oil bottling processes, in the processing of well drilling muds, and in general in any application needing entrained air information.
Anton-Paar's CARBO 2100 CO2 analyzer employs a patented impeller method which is said to make it significantly faster that other commercially available systems for measuring and monitoring tasks and also for regulating the CO2 content of process liquids during production runs in the beer and soft drink industry.
It is believed that all of these instruments adopt a common approach, using Boyle's Law. Boyle's law is given by the formulaP1V1=P2V2  (1)where V1 and V2 are the volumes of the free gas in the liquid at two different pressures, P1 and P2, respectively. Being a “two-point measurement”, this common approach measures the volume difference ΔV=V1−V2 between P1 and P2, and calculates the volumes of free gas, V1 and V2, from Boyle's Law as                               V          1                =                                                                              P                  2                                ⁢                Δ                ⁢                                                                  ⁢                V                                                              P                  2                                -                                  P                  1                                                      ⁢                                                  ⁢            and            ⁢                                                  ⁢                          V              2                                =                                                    P                1                            ⁢              Δ              ⁢                                                          ⁢              V                                                      P                2                            -                              P                1                                                                        (        2        )            More general formulas, which correlate the volumes of free gas with the pressures being acted upon, can be derived from the Ideal Gas Law asP1V1=n1RT1  (3)andP2V2=n2RT2  (4)where R is the gas constant, and n1, T1 and n2, T2 are moles of free gas and temperatures at P1 and P2, respectively. In the case of n1=n2 and T1=T2, equations (3) and (4) can be simplified to the equation of Boyle's Law given in (1). Hence, Boyle's Law is, in fact, a special case of the Ideal Gas Law and is valid only if the moles of free gas and temperatures at P1 and P2 are kept constant.
In practice, a portion of free gas, however, will be dissolved into the liquid. The solubility of gas is, as a general rule, proportional to the gas pressure as stated in Henry's LawP=Hnd  (5)where P, H, nd are the pressure of the gas being dissolved, the constant of Henry's Law, and moles of dissolved gas, respectively. This unquestionably makes n1≠n2 between P1 and P2, causing a violation of Boyle's Law. Therefore, using Boyle's Law for a “two-point measurement” is an unreliable approximation and can cause a significant amount of error, especially when the pressure difference between the two points becomes large.
To cure this error, there have been some attempts to use Henry's Law to compensate for the amount of the dissolved gas. This approach, however, is generally impractical, inasmuch as the constants of Henry's Law are not available for many process liquids, particularly for those containing multiple-components such as coating slurries. Using the known constant of one liquid to approximate the constant of the others may potentially introduce a considerable amount of error, because the solubility of gases such as air changes dramatically from liquid to liquid. The solubility of air in isooctane at standard temperature and pressure, for example, is more than 100 times higher than the solubility of air in water.